1. Field of the Invention
The present invention relates to a radar apparatus having a structure in which a beat signal generated by mixing transmission and reception signals of a radar wave is processed to detect a target reflecting the radar wave.
2. Description of Related Art
There is known a vehicle-mounted radar apparatus configured to transmit a radar wave having been frequency-modulated in a triangular wave shape along a time axis at a predetermined measurement cycle, receive the radar wave reflected from a target, frequency-analyze a beat signal generated by mixing the transmission and reception signals of the radar wave to provide a power spectrum, and detect a peak frequency at which the intensity (power) of the power spectrum peaks as a target candidate. If the target candidate continues to be detected for one or more measurement cycles, the target candidate is recognized as a target such as a preceding vehicle or a roadside object when a predetermined condition is met.
When a vehicle equipped with such a radar apparatus is running in a closed space such as a tunnel, the radar apparatus receives, in addition to the radar wave reflected by a target, the radar wave reflected by the target and further by the tunnel wall or tunnel ceiling, or equipment in the tunnel such as a jet fan.
In this case, the radar apparatus may detect the peak frequency of the reflected radar wave as a target candidate although it does not exist, that is, although it is a so-called ghost.
To deal with this problem, it is proposed to change a detection threshold when the vehicle runs in a tunnel. For example, refer to Japanese Patent Application Laid-open No. 2008-51771. The radar apparatus disclosed in this patent document is configured to determine that the vehicle is running in a tunnel if the illuminance level outside of the vehicle measured by an illuminance sensor mounted on the vehicle is below a predetermined threshold value.
However, since the illuminance level measured by the illuminance sensor is always below the threshold value when the vehicle runs in a dark environment, for example, when the vehicle runs at night, the radar apparatus may erroneously determine that the vehicle is running in a tunnel although the vehicle is running in an open space. In this case, since the detection threshold is changed wrongly, the accuracy of target recognition is degraded.
That is, the conventional radar apparatus as described in the above patent document cannot accurately determine whether or not the vehicle is running in a closed space, and accordingly cannot accurately recognize a target in a dark environment.